The continents can be separated by one or more of the following barriers:

1. The Everstorm

A grand, unending storm of epic proportions guaranteed to sink any ship or wreck any flying vessel. Based on the Everstorm from The Legend of Korra and the Everstorm from Words of Radiance by Brandon Sanderson, this barrier lasts until some quest is completed by the heroes.

2. The Great Devourer

A gargantuan and monstrous being lurks in the depths between the continents, preying on any vessels foolish enough to try their luck on or over the seas. Based on Jormungandr the World Serpent from Norse mythology (said to guard the boundaries of the world), this monster can only be slain when the gods do battle at the end of the old world or by a hero wielding the hammer of thunder.

3. The Far Shore

The continents are separated by a vast distance, beyond the means for any ship or flying vessel to cross without running out of supplies. Based on the real world distance between Europe and America before the days of Columbus, this distance is overcome by the creation of new technology blessed by the gods (a cornucopia, refrigeration, better preservatives, spatial chests, etc).

4. The Endless Maze

A mysterious confusion assails any who traverse the seas beyond a certain point, and only the fortunate survive to return to their ports of origin. Based on real world lack of proper navigation skills (the later development of longitude, for example), this mental assault can only be relieved by the gods themselves.

There's no need for physical walls unless you want such.

The continents can be separated by one or more of the following barriers:

1. The Everstorm

A grand, unending storm of epic proportions guaranteed to sink any ship or wreck any flying vessel. Based on the Everstorm from The Legend of Korra and the Everstorm from Words of Radiance by Brandon Sanderson, this barrier lasts until some quest is completed by the heroes.

2. The Great Devourer

A gargantuan and monstrous being lurks in the depths between the continents, preying on any vessels foolish enough to try their luck on or over the seas. Based on Jormungandr the World Serpent from Norse mythology (said to guard the boundaries of the world), this monster can only be slain when the gods do battle at the end of the old world or by a hero wielding the hammer of thunder.

3. The Far Shore

The continents are separated by a vast distance, beyond the means for any ship or flying vessel to cross without running out of supplies. Based on the real world distance between Europe and America before the days of Columbus, this distance is overcome by the creation of new technology blessed by the gods (a cornucopia, refrigeration, better preservatives, spatial chests, etc).

4. The Endless Maze

A mysterious confusion assails any who traverse the seas beyond a certain point, and only the fortunate survive to return to their ports of origin. Based on real world lack of proper navigation skills (the later development of longitude, for example), this mental assault can only be relieved by the gods themselves.